The invention relates to an optical transmission system comprising a semiconductor laser diode which is coupled to a multimode transmission fiber via a coupling path.
When high quality laser diodes are combined in an optical transmission system with multimode fibers, modal noise is a very serious problem. This problem is particularly annoying in transmission systems using analog modulation. The large coherence length of these laser diodes creates in the transmission fiber, at large distances from the laser, time-varying intensity distributions across the fiber cross-section which may cause intensity modulation of the optical signal behind a nonideal coupling. Because of their shape these intensity distributions are called speckle patterns, as described in, for example, the article entitled "The Phenomenon of Modal Noise In Analogue and Digital Optical Fibre Systems" by R. E. Epworth. (Proceedings of the Fourth European Conference on Optical Communication, Sept. 12-15, 1978, Genoa, pages 492-501).
In addition, reflection noise is a problem in an optical transmission system. This type of noise is the result of a nonideal coupling which reflects a portion of the intensity-varying light and returns it into the laser diode. The varying, reflected light which reaches the laser may be coupled into the fiber to greater extent by the active medium of the laser, and it can consequently considerably disturb the originally desired optical signal. This problem is described in, for example, the article entitled "Model For Return-Beam-Induced Noise Generation In GaAlAs Semiconductor Lasers" by Y. Hirose et al (Electronics Letters, Mar. 13th, 1980, Volume 16, No. 6, pages 202-204.